The present invention relates to a hollow cam shaft (camshaft) having a structure in which an end piece is pressure-welded to a hollow shaft particularly capable of enclosing burr or flush generated at a time when the end piece is pressure-welded to the hollow shaft. Further, in the above meaning, the hollow cam shaft of the present invention may be referred to as end piece pressure welded type hollow cam shaft or hollow cam shaft having an end piece pressure-welded structure.
A cam shaft constituting one element of an engine of an automobile, for example, is usually manufactured by assembling and fixing a hollow shaft and various pieces such as cam piece, journal piece, end piece and the like. In the prior art, the Japanese Patent Laid-open Publication No. SHO 54-86456 discloses a method of fixing an end piece to a hollow shaft by means of brazing and soldering, and on the other hand, the Japanese Patent Laid-open Publication No. HEI 7-167152 discloses a method of fixing an end piece to a hollow shaft by fitting, under pressure, i.e. press-fitting, the end piece into the hollow shaft. Furthermore, a fixing method utilizing a welding process is also known.
However, the method by means of brazing or welding requires consumable goods or materials such as filler material (metal), flux, sealed gas or other, and furthermore, in the press-fitting method, it is necessary to preliminarily carry out a machine working to the end piece and the hollow shaft with high performance. Accordingly, in these conventional methods, there are provided problems of simplifying processes or steps, reducing cost and so on.
Taking the above matters into consideration, the inventors of the subject application attempted to fix both the end piece and the hollow shaft by means of friction welding, which has been well known in the prior art. The friction welding method is a method of abutting materials to be joined, applying a pressure thereto, performing a mechanical relative movement at their mating surfaces and then joining these materials by a friction heat generated through the relative movement thereof. The joining method by means of such friction welding does not require a consumable goods or products such as filler material used in the blazing and soldering method mentioned above. Furthermore, this joining method does not preliminarily require a precise mechanical (machine) working of the end piece and the hollow shaft. Accordingly, this joining method has advantages in comparison with the other methods mentioned above.
FIG. 6 is a sectional view showing one example of an end piece pressure-welding type hollow cam shaft 61 which was manufactured by the inventors through the frictional welding.
With reference to FIG. 6, an end piece 63 is fixed to a hollow shaft 62 by means of the friction welding, burrs 65 and 66 are formed to both inner and outer peripheral surface sides of a pressure-welding (welded) portion 64. Although the outer burr 66 formed to the outer peripheral surface side can be easily removed through a simple mechanical working, it is very difficult to remove the inner burr 65 formed to the inner peripheral surface side. When the hollow cam shaft 61 with the inner burr 65 being not removed, particularly, in a case where an inner hollow portion inside the cam shaft is utilized as an oil passage, there is a fear of the falling of the burr 65 and the invading of foreign materials into an inside of an engine, which may give a significant damage to a valve moving mechanism and, hence, to the whole structure of the engine.